WASHINGTON — President Obama, after months of sitting on the sidelines of the rancorous contest to succeed him, is now ready to aggressively campaign for Hillary Clinton, starting with a formal endorsement of her candidacy as early as this week.

The White House is in active conversations with Mrs. Clinton’s campaign about how and where the president would be useful to her, according to senior aides to Mr. Obama.

Advisers say that the president, who sees a Democratic successor as critical to his legacy, is impatient to begin campaigning. They say he is taking nothing for granted.

“I want us to run scared the whole time,” Mr. Obama told a group of donors on Friday night in Miami.

It has been decades since a second-term president enjoyed the popularity to make him a potent force on the campaign trail and also an invitation from the candidate running to succeed him to be a major presence there.

Mr. Obama’s approval rating was at 50 percent in this month’s New York Times/CBS poll, and strategists close to Mrs. Clinton said they would be eager to have his participation as the general election unfolds. (In contrast, President George W. Bush’s approval rating was at 20 percent in a Gallup poll just before the November 2008 election, and he rarely appeared that year with Senator John McCain of Arizona, the Republican nominee.)

Mr. Obama is particularly enthusiastic, aides said, about taking on Mr. Trump. The Republican candidate has personally offended the president with his conduct on the campaign trail — Mr. Trump referred to a black supporter on Friday in one of his crowds as “my African-American” — and as the most visible champion of the “birther” conspiracy theories that falsely hold that Mr. Obama was born in Kenya rather than Hawaii.

Should Mrs. Clinton do well enough in the primaries on Tuesday to give her sufficient delegates to claim the Democratic nomination, Mr. Obama is likely to move swiftly to make a case for her.

“He has indicated he wants to spend a lot of time on the campaign trail, so when it’s time to do that, we’ll go out guns ablazing,” Jennifer Psaki, Mr. Obama’s communications director, said in an interview. “We are actively thinking through how to use the president on the campaign trail — what works for the nominee, what works for him, and how to utilize his strengths and his appeal.”

Jennifer Palmieri, Mrs. Clinton’s communications director, said that Mrs. Clinton hoped to earn Mr. Obama’s endorsement and his active participation in the campaign during the summer and fall.

“There’s no one better to lay out the two paths voters will face in the fall elections,” Ms. Palmieri said, “and he is particularly strong at making the economic argument for her.”

Advisers to Mr. Obama and Mrs. Clinton believe that the president, who ran against Mrs. Clinton in a sometimes nasty primary in 2008, can be a persuasive voice for voters who may find her difficult to relate to or who have supported the more liberal stances of Senator Bernie Sanders.

“As former opponents, they have an amazing story,” Ms. Palmieri said, adding that it would be “hard to imagine a more convincing advocate for her.”

The White House argues that Mr. Obama could help Mrs. Clinton appeal to independent voters — particularly suburban independent women — in the Midwest, notably its northern stretch including Michigan, Minnesota and Wisconsin. Mr. Obama won all three states in 2012 in part by painting Mitt Romney, the Republican nominee, as a corporate raider who would shut down factories and move jobs overseas.

Officials in the White House and close to Mrs. Clinton’s team point to the president’s appeal to African-American and young voters — groups who favor him more than her — as helpful to turning those groups out for her in November. African-American and young voters will play a crucial role, the aides said, in battlegrounds like Florida, Virginia, Pennsylvania and Ohio. Mr. Obama won all four states in 2012.

The process comes after months of more informal contact between the White House and the Clinton and Sanders campaigns and is still in its early stages. Advisers to Mr. Obama, speaking on the condition of anonymity about his plans because Mrs. Clinton has yet to secure the Democratic nomination, said the president had not begun drafting his address for next month’s Democratic National Convention or finalized any particular campaign itinerary.

The first look at Mr. Obama’s campaign for Mrs. Clinton came this past Wednesday, when he used a speech in Elkhart, Ind. — the first American city he visited as president in 2009 — to lay out an argument against the Republican economic agenda and, without naming him, an indictment of the presumptive Republican nominee, Donald J. Trump.

Mr. Obama signaled that day that he would soon throw his full support behind Mrs. Clinton. He said Tuesday’s primary contests, including California and New Jersey, would yield a “pretty good sense of who the nominee will end up being.”

“He’s been very respectful of both of them and careful not to put his thumb on the scale, but at some point, the verdict is the verdict, and that point is almost certainly Tuesday, which is what he was saying,” said David Axelrod, a former senior adviser to Mr. Obama’s presidential campaign and at the White House. “I expect that he’ll be a force for trying to move this process along so the party can consolidate and unify.”

White House and Clinton campaign officials declined to pinpoint any timing for an endorsement. But Mr. Obama has scheduled a trip to New York on Wednesday to attend Democratic fund-raisers, placing him near Mrs. Clinton’s Brooklyn campaign headquarters the day after Tuesday’s balloting.

Pete Brodnitz, a veteran Democratic pollster, said Mr. Obama’s position and popularity makes him ideally suited to counter Mr. Trump’s message of economic anxiety. But he cautioned that the president’s approval rating, and his effectiveness on Mrs. Clinton’s behalf, could plunge if he engages in a bitter, personal back-and-forth with the Republican nominee.

“No one really rewards politicians fighting each other in a general election,” Mr. Brodnitz said. “The party is full of attack dogs. I think the president is the one person who does not need to be an attack dog if he can make people feel just a little bit better about our economic future.”

Mrs. Clinton’s advisers believe that Mr. Obama himself is the most powerful voice they can have driving home her argument that Mr. Trump is unfit to be president.

“There are only five people on the planet who understand the demands of the presidency in all its aspects, and as such, he has concerns not just about Trump’s policies — although those are big — but about whether he can meet the demands of the office,” Mr. Axelrod said. “The best role that he can play is to be the guy who provides reality checks, but more importantly focuses people on the stakes of this election and the stark differences between the candidates.”

Mr. Obama’s regular presence on the campaign trail could have some downsides. Ed Rollins, who ran Ronald Reagan’s re-election campaign in 1984, said Mrs. Clinton could be overshadowed by the kind of charisma that Mr. Obama showed on the stump in 2008 and 2012.

“She’s tired,” said Mr. Rollins, who is supporting a super PAC backing Mr. Trump. “She’s not a very good candidate. There’s a high risk there of her getting overshadowed.”